As a memory (semiconductor memory device) that is used in place of a flash memory whose memory cell has been approaching a limitation in its miniaturization, a resistance change memory that records information by changing an electric resistance (hereinafter, simply referred to also as “resistance”) of a recording film has been studied. As an example of such a resistance change memory, a phase change memory provided with a memory cell having a structure in which a recording film made of a chalcogen compound such as Ge2Sb2Te5 is sandwiched between two electrodes has been extensively studied.
A phase change memory having the above-mentioned recording film made of a chalcogen compound records information by utilizing the fact that resistance of the recording film differs depending on whether the chalcogen compound is in an amorphous state or a crystalline state. More specifically, the above-mentioned chalcogen compound has a high resistance in an amorphous state (high resistance state) and has a low resistance in a crystalline state (low resistance state). Therefore, reading of information recorded in the recording film is carried out by measuring an electric current that flows in the recording film when a voltage is applied between the two electrodes sandwiching the recording film and determining whether the recording film is in the high resistance state or in the low resistance state.
Moreover, recording of information onto the recording film, that is, writing thereof is carried out by applying a voltage between the two electrodes sandwiching the recording film to pass an electric current through the recording film so as to generate a Joule heat, thereby changing the chalcogen compound forming the recording film between the amorphous state (high resistance state) and the crystalline state (low resistance state). As the operations for the writing described above, a resetting operation and a setting operation are carried out. In the resetting operation, the recording film is changed from the low resistance state (crystalline state) to the high resistance state (amorphous state), and for example, this operation is carried out through a process in which, after the chalcogen compound forming the recording film is once melted by passing a large electric current through the recording film for a short period of time, the chalcogen compound is then rapidly cooled by quickly reducing the electric current. On the other hand, in the setting operation, the recording film is changed from the high resistance state (amorphous state) to the low resistance state (crystalline state), and for example, this operation is carried out through a process in which, by passing an electric current through the recording film for a long period of time, the chalcogen compound forming the recording film is maintained at a crystallization temperature at which the chalcogen compound is crystallized.
In accordance with the above-mentioned writing principle, since an electric current required for changing the chalcogen compound between the high resistance state and the low resistance state becomes smaller as the memory cell is miniaturized, the phase change memory is suitable for the miniaturization of the memory cell, so that its studies have been extensively carried out.
In order to further improve performances of the semiconductor memory device made up of the phase change memory like this, the phase change memory provided with a recording film having a super lattice structure has been proposed (see Japanese Patent Application Laid-Open Publication No. 2010-263131 (Patent Document 1) and R. E. Simpson et al, “Interfacial phase-change memory”, Nature Nanotechnology, Vol. 6, pp. 501-505, 2011 (Patent Document 2)). In this phase change memory, the recording film is not made of generally used Ge2Sb2Te5, but is configured to have a super lattice structure in which very thin crystal layers made of GeTe and very thin crystal layers made of Sb2Te3 are alternately stacked repeatedly, and by reversibly moving positions of the constituent atoms of the crystal layers made of GeTe, the electric resistance of the recording film is changed.